1. Field of the Invention
This invention relates to a heavy duty pneumatic radial tire, and more particularly to a heavy duty pneumatic radial tire for use in truck and bus in which cracks liable to be created at an end portion of a cord layer in a belt and growth thereof are controlled and minimized to favorably suppress the occurrence of separation failure due to the crack growth when the tire is applied to a heavy duty vehicle such as truck and bus.
2. Description of Related Art
It is well-known that as the running distance of the pneumatic radial tire mounted onto the heavy duty vehicle such as small-large size truck and bus increases, cracks are created at a cord end of a narrow-width cord layer in cross cord layers having different widths, cords of which layers being crossed with each other at a relatively small cord angle with respect to an equatorial plane of the tire, among plural cord layers constituting the belt. The crack initially created is minute like xe2x80x9cpeckingxe2x80x9d of the cord end, but such a minute crack propagates to a big crack along the cord as the running distance of the tire increases. When the crack growth progresses in a certain extent, the front end of the crack grows toward a cord end of the adjoining cord and finally cracks are connected to each other over substantially a full periphery of the cord layer along a side edge of the cord layer. If the crack growth is advanced to this stage, a running time is not taken so much until the separation failure is created between the cross cord layers.
Accordingly, a period ranging from the occurrence of minute crack at the cord end to the growth of connected crack along the side edge of the cord layer on its periphery controls a service life of the tire through belt separation failure. For this end, there are proposed various improving structures of the tire directed to the prolongation of tire running time (running distance) though the connection of cracks is unavoidable.
As the improving structure, JP-A-4-183605 proposes a heavy duty pneumatic radial tire wherein a cushion rubber is arranged between end portions of second and third cross steel cord layers counted from a side of a carcass in a belt comprised of 3 to 4 steel cord layers and an upper side of an end portion of the third layer is covered with a rubber layer having a thickness of not less than 1.5 mm to overlap over a distance of not less than 20 mm from the end of the layer and the rubber layer is made from rubber having a JIS hardness of 65-75 and a modulus at 300% elongation of 130-200 kgf/cm2.
Furthermore, JP-A-4-252705 discloses a heavy duty pneumatic radial tire wherein a belt edge cushion rubber is interposed between end portions of two cross steel cord layers in a belt comprised of four steel cord layers and such end portion is covered with a rubber sheet having a thickness of 1.0-3.0 mm and 50% modulus of the rubber sheet is smaller than that of a coating rubber for the steel cord layer but larger that that of a base tread rubber. Moreover, JP-A-6-320906 proposes a heavy duty pneumatic radial tire wherein a belt comprised of four steel cord layers is arranged between a base tread rubber of low modulus and a carcass and a cushion rubber is disposed between end portions of second and third cross cord layers counted from a side of the carcass and the end portion is covered with a rubber sheet having a thickness of 0.8-3.3 mm likewise the case of JP-A-4-252705 and further the rubber sheet has a JIS hardness lower than those of rubber for the second and third cord layers and 50% modulus larger than that of the base tread rubber.
However, the effect of sufficiently controlling the crack growth under severe service conditions of heavy loading, running speed and the like can not be attained even by any improving structure of the belt described in these prior proposals, so that belt separation failure is naturally liable to be caused even in these new tires. Therefore, even if the tire is recapped, it is obliged to prematurely create such a failure. Although this type of the heavy duty pneumatic radial tire particularly tends to be important in the recapping adaptability, even when the above new tire is completely run, the occurrence of the big crack unsuitable for the recapping is frequently observed at the end portion of the belt. Therefore, it is demanded to further improve the crack resistance and resistance to crack growth at the end portion of the belt and hence considerably improve the resistance to belt separation.
It is, therefore, an object of the invention to provide a heavy duty pneumatic radial tire having an excellent long service life in which the crack resistance and resistance to crack growth at the end portion of the cord layer constituting the belt, particularly narrow-width cord layer in cross cord layers are sufficiently enhanced even under severer service conditions of the tire while sufficiently maintaining the rigidity-strengthening function inherent to the belt of the radial tire to thereby highly improve the resistance to separation in a new tire and also improve the recapping adaptability.
According to a first aspect of the invention, there is the provision of a heavy duty pneumatic radial tire comprising a radial carcass toroidally extending between a pair of bead cores embedded in a pair of bead portions and a belt superimposed about an outer periphery of the carcass to reinforce a tread portion and comprised of at least three rubberized cord layers, cords of two adjacent layers among these layers being crossed with each other at an acute cord angle with respect to an equatorial plane of the tire to form cross cord layers, in which an outer cord layer of the cross cord layers in a radial direction of the tire has a width narrower than that of an inner cord layer, and such a narrow-width cord layer is provided at its end portion with an end cover rubber covering the end portion and having a 100% modulus larger than that of a coating rubber for the narrow-width cord layer, and a space cushion rubber having a 100% modulus smaller than that of a coating rubber for each of the cross cord layers is disposed between the end portions of the cross cord layers so as to separate these end portions from each other exceeding the thickness of the end cover rubber.
In the first aspect, the position relation of the cross cord layers among the cord layers constituting the belt with respect to the radial carcass is not particularly critical, but it is desirable that the cross cord layers are second and third layers or third and fourth layers counted from a side of the radial carcass. Further, the end cover rubber indicates a sheet-shaped rubber covering each of the end portions of the narrow-width cord layer from both surfaces thereof in the radial direction of the tire otherwise specified. Also, the space cushion rubber is a rubber layer taperingly extending from an end of the narrow-width cord layer or the end cover rubber toward inside and outside of the tire. Moreover, the term xe2x80x9c100% modulusxe2x80x9d used herein means a tensile stress at 100% elongation (kgf/cm2) measured according to a tensile test method of a vulcanized rubber defined in JIS K-6251(1993).
In a preferable embodiment of the first aspect, the end cover rubber has the 100% modulus larger by at least 1.2 times than that of the coating rubber for the narrow-width cord layer, and the space cushion rubber has the 100% modulus smaller by at least 0.95 times than that of the coating rubber for the cross cord layers.
In another preferable embodiment of the first aspect, the 100% modulus (M5) (kgf/cm2) of the space. cushion rubber satisfies a relationship of M5xe2x89xa6Mxxe2x88x92(M4xe2x88x92Mx)xc3x97(G4E/G5E) with respect to 100% modulus (M4) (kgf/cm2) of the end cover rubber, 100% modulus (Mx) (kgf/cm2) of the coating rubber for the cross cord layers, gauge (G4E) (mm) of the end cover rubber at the inside of the end of the narrow-width outer cord layer in the radial direction of the tire and gauge (G5E) (mm) of the space cushion rubber existing on a vertical line drawn from an inner end of the narrow-width cord layer in the radial direction of the tire to the inner cord layer of the cross cord layers.
In the other preferable embodiment of the first aspect, a width w (mm) of the end cover rubber at least located at the inside thereof in the radial direction of the tire satisfies a relationship of wxe2x89xa7(50 mm/N)xc3x97sin xcex8 with respect to an inclination angle xcex8 defined between a normal line drawn from a corner of any cord end in the narrow-width cord layer at its developed plane view to a center axis line of a cord adjacent thereto and a straight line passing through cord ends of this layer and an end count N of the narrow-width cord layer per 50 mm as measured in a direction perpendicular to the cords of the layer, and an inner end of the end cover rubber having the width w in the widthwise direction of the belt is located in a region of arranging the space cushion rubber.
In a further preferable embodiment of the first aspect, the end cover rubber covers the surface of the end portion of the narrow-width cord layer facing to the space cushion rubber and extends over this end portion toward the outside of the tire. This can be said to be simple as compared with the end cover rubber defined in the aforementioned preferable embodiments.
According to a second aspect of the invention, there is the provision of a heavy duty pneumatic radial tire comprising a radial carcass toroidally extending between a pair of bead cores embedded in a pair of bead portions and a belt superimposed about an outer periphery of the carcass to reinforce a tread portion and comprised of at least three rubberized cord layers, cords of two adjacent layers among these layers being crossed with each other at an acute cord angle with respect to an equatorial plane of the tire to form cross cord layers, in which an outer cord layer of the cross cord layers in a radial direction of the tire has a width narrower than that of an inner cord layer, and such a narrow-width cord layer is provided at its end portion with a double end cover rubber covering the end portion and comprised of an outer rubber and an inner rubber having different compositions, and the outer rubber has a 100% modulus larger than that of the inner rubber, and the inner rubber has a 100% modulus larger than that of a coating rubber for the narrow-width cord layer.
This double end cover rubber is a sheet-shaped rubber covering each of the end portions of the narrow-width cord layer from inside toward outside in the radial direction of the tire likewise the first aspect of the invention otherwise specified.
According to a third aspect of the invention, there is the provision of a heavy duty pneumatic radial tire comprising a radial carcass toroidally extending between a pair of bead cores embedded in a pair of bead portions and a belt superimposed about an outer periphery of the carcass to reinforce a tread portion and comprised of at least three rubberized cord layers, cords of two adjacent layers among these layers being crossed with each other at an acute cord angle with respect to an equatorial plane of the tire to form cross cord layers, in which an outer cord layer of the cross cord layers in a radial direction of the tire has a width narrower than that of an inner cord layer, and such a narrow-width cord layer is provided with an end cover rubber covering each of the end portions of this layer, and at least one of outer portion and inner portion of the end cover rubber in the radial direction of the tire forms a wavy form in a direction perpendicular to the cords arranged in the narrow-width cord layer, and a height between a bottom and a peak of the wavy surface is within a range of 0.05-0.25 mm.
In this case, the peak of the wavy form substantially corresponds with the position of the central axis line of the cord embedded in the narrow-width cord layer, while the bottom of the wavy form substantially corresponds with a central position between mutual adjoining cords. Moreover, an amplitude of the wavy surface in the end cover rubber is preferable to be within a range of 0.07-0.20 mm.
According to a fourth aspect of the invention, there is the provision of a heavy duty pneumatic radial tire comprising a radial carcass toroidally extending between a pair of bead cores embedded in a pair of bead portions and a belt superimposed about an outer periphery of the carcass to reinforce a tread portion and comprised of at least three rubberized cord layers, cords of two adjacent layers among these layers being crossed with each other at an acute cord angle with respect to an equatorial plane of the tire to form cross cord layers, in which an outer cord layer of the cross cord layers in a radial direction of the tire has a width narrower than that of an inner cord layer, and the cross cord layers are provided with a space cushion rubber separating end portions of these layers and extending from an end of the narrow-width cord layer toward the outside of the tire, and the space cushion rubber is a rubber composite comprised of an inner rubber extending so as to separate the end portions of the layer from each other and an outer rubber extending from the inner rubber toward the outside of the tire and having at least two different rubber compositions, and 100% moduli Mxxe2x80x2, M6i and M6u (kgf/cm2) of a coating rubber for the narrow-width outer cord layer, inner rubber (6i) and outer rubber (6u) establish a relationship of M6uxe2x89xa6M6iMxxe2x80x2.
When the space cushion rubber is comprised of two rubbers, the position of joint face between the inner rubber 6i and the outer rubber 6u may be located on either an end of the narrow-width cord layer or an end of the end cover rubber provided on the end portion of the layer as a standard point or may be within a region slightly separated from the above standard point toward the inside or outside of the tire.
According to a fifth aspect of the invention, there is the provision of a heavy duty pneumatic radial tire comprising a radial carcass toroidally extending between a pair of bead cores embedded in a pair of bead portions and a belt superimposed about an outer periphery of the carcass to reinforce a tread portion and comprised of at least three rubberized cord layers, cords of two adjacent layers among these layers being crossed with each other at an acute cord angle with respect to an equatorial plane of the tire to form cross cord layers, in which an outer cord layer of the cross cord layers in a radial direction of the tire has a width narrower than that of an inner cord layer, and such a narrow-width cord layer is provided with a space rubber covering an outer peripheral surface side of an end portion of the layer and projecting outward from the end of the layer in the widthwise direction thereof, and an inner peripheral surface of the projected space rubber is located outward from an extrapolated extending surface of the outer peripheral surface of the narrow-width cord layer in the radial direction of the tire, and the space rubber has a 100% modulus larger than that of a coating rubber for the narrow-width cord layer.
If the end cover rubber is not provided on the end portion of the narrow-width cord layer, the space rubber is arranged so as to directly contact with the end portion, while if the end cover rubber is provided on the end portion of the layer, the space rubber is arranged so as to directly contact with the outer periphery of the end cover rubber to separate the end portion from a tread rubber. The space rubber is a flat sheet having a taper on its end portion and may be triangular or trapezoidal in section, if necessary.
According to a sixth aspect of the invention, there is the provision of a heavy duty pneumatic radial tire comprising a radial carcass toroidally extending between a pair of bead cores embedded in a pair of bead portions and a belt superimposed about an outer periphery of the carcass to reinforce a tread portion and comprised of at least three rubberized cord layers, cords of two adjacent layers among these layers being crossed with each other at an acute cord angle with respect to an equatorial plane of the tire to form cross cord layers, in which the cross cord layers are provided with a side space rubber extending between ends of these layers located at the same side, and the side space rubber has a 100% modulus smaller than that of a coating rubber for the cross cord layers.
The side space rubber serves to separate the ends of the cross cord layers from a tread rubber likewise the aforementioned space rubber covering the outer peripheral surface side of the end portion of the narrow-width cord layer. Therefore, it is required that an inner portion of the side space rubber in the radial direction of the tire covers the end of the inner cord layer among the cross cord layers, so that it is desirable to arrange the side space rubber to extend over the end of the inner cord layer.
According to a seventh aspect of the invention, there is the provision of a heavy duty pneumatic radial tire comprising a radial carcass toroidally extending between a pair of bead cores embedded in a pair of bead portions and a belt superimposed about an outer periphery of the carcass to reinforce a tread portion and comprised of at least three rubberized cord layers, cords of two adjacent layers among these layers being crossed with each other at an acute cord angle with respect to an equatorial plane of the tire to form cross cord layers, in which an outer cord layer of the cross cord layers in a radial direction of the tire has a width narrower than that of an inner cord layer, and at least a coating rubber for such a narrow-width cord layer among the cord layers constituting the belt extends over an end of the cord in this layer toward the outside of the tire, and a distance from the end of the cord to an outer end of the coating rubber is within a range of 0.05-0.70 mm.
In general, cords in the cord layer constituting the belt slightly protrude from the coating rubber at an unvulcanized state and the cord layer is built up into an unvulcanized tire at such a state, which is then vulcanized to prepare a product tire in which the ends of the cords are maintained at a state of slightly protruding from the coating rubber. On the contrary, the tire of the seventh aspect is characterized by surely and completely enveloping the cord end with the coating rubber as compared with the above conventional tire, which develops the effect of attaining the object of the invention. It is desirable that the distance from the end of the cord to the outer end of the coating rubber is within a range of 0.10-0.50 mm.
The first to seventh aspects develop the effect of attaining the object of the invention individually, but may be adequately and freely combined in accordance with the severity of service conditions.
In order to favorably support the effect of attaining the object of the invention, it is desirable that a belt under-cushion rubber is disposed between the carcass and an end portion of the cord layer located close to the carcass and has a 100% modulus corresponding to 0.3-0.7 times the 100% modulus of a coating rubber for this cord layer, or that either organic fiber cord or steel cord is applied to the narrow-width cord layer among the cross cord layers and the distance between the cords in a given end count is unequal.